Method of improving the power cycle efficiency of a steam turbine for supercritical steam conditions

ABSTRACT

A method for improving the efficiency of a power cycle in a steam turbine for initial supercritical steam conditions in which the working fluid is expanded from the initial supercritical steam conditions to a lower pressure which is, however, higher than the critical pressure. The working fluid is divided into two streams, one of which is cooled down in a high temperature regenerative heat exchanger for yielding its heat to feed water already previously heated in regenerative feed heaters. The other stream is expanded in the turbine in a conventional manner, and thereafter the two streams are mixed before entering the high temperature regenerative heat exchanger and boiler. The high temperature regenerative heat exchanger is sub-divided into a number of sections, and the mass flow rate of the heating medium in predetermined sections is changed in a step manner by directing a part of the working fluid from a predetermined point of the power cycle to a predetermined section of the high temperature regenerative heat exchanger.

Szewalski i 1 June 17, 1975 METHOD OF IMPROVING THE POWER CYCLE EFFICIENCY OF A STEAM TURBINE FOR SUPERCRITICAL STEAM CONDITIONS [75] Inventor: Robert Szewalski.

Gdansk-Wrzeszcz. Poland [73] Assignee: Polska Akademia Nauk Instytut Maszyn Przeplywowych. Gdansk. Poland [22] Filed: June 8. 1973 [21] Appl. No.; 368.122

[30] Foreign Application Priority Data June 10. 1972 Poland 155928 [52] US. Cl 60/647; 60/678 [51] Int. Cl. F01k 7/40 [58] Field of Search 60/647. 678

[56] References Cited UNITED STATES PATENTS 2,991,620 7/l96l Nekolny 60/107 X 3.292.372 l2/l966 Michel 60/107 X 3.683.621 8/1972 Szewalski 60/73 Primary Exanziner-Martin P. Schwadron Assistant ExaminerAllen M. Ostrager [57] ABSTRACT A method for improving the efficiency of a power cycle in a steam turbine for initial supercriticalsteam conditions in which the working fluid is expanded from the initial supercritical steam conditions to a lower pressure which is. however. higher than the critical pressure. The working fluid is divided into two streams, one of which is cooled down in a high temperature regenerative heat exchanger for yielding its heat to feed water already previously heated in regenerative feed heaters. The other stream is expanded in the turbine in a conventional manner. and thereafter the two streams are mixed before entering the high temperature regenerative heat exchanger and boiler. The high temperature regenerative heat exchanger is sub-divided into a number of sections. and the mass flow rate of the heating medium in predetermined sections is changed in a step manner by directing a part of the working fluid from a predetermined point of the power cycle to a predetermined section of the high temperature regenerative heat exchanger.

4 Claims, 2 Drawing Figures PATENTEDJLIN 17 I975 RVs/c5 FIG.

METHOD OF IMPROVING THE POWER CYCLE EFFICIENCY OF A STEAM TURBINE FOR SUPERCRITICAL STEAM CONDITIONS The present invention relates to a method of improving the power cycle efficiency of a steam turbine for supercritical steam conditions.

In US. Pat. No. 3,683,621 there is described a method of increasing the efficiency of the power cycle ofa steam turbine for supercritical steam conditions, in accordance with which the working fluid is expanded from the initial conditions to a lower pressure which is however always higher than the critical pressure, and then the stream of the said working fluid is divided into two streams. One of the streams is cooled in a high temperature heat exchanger and yields heat to feed water preliminary heated already in regenerative feed heaters. The other stream of the working fluid is expanded in a conventional turbine with application of interstage reheat, and multistage regenerative feed water heating by means of condensing extraction steam. Next, the

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above mentioned two streams of the working fluid are brought together again before entering the high temperature heat exchanger and the boiler.

The essential feature of the method in US. Pat. No. 3,683,621 is the superposition of two thermodynamic cycles for supercritical steam conditions, one of which is basically a conventional cycle while the other for identical initial steam conditions is confined exclusively to the range of supercritical pressures, whereas the above mentioned two cycles are thermally connected by means of a high temperature heat exchanger. The above method enables obtaining in a steam turbine power plant an efficiency higher by approximately 7 percent in comparision with a conventional power cycle performed within the same range of temperatures.

It has been found that there exists the possibility of further improvement of the power cycle efficiency in the method in US. Pat. No. 3,683,621 when ensuring approximately constant temperature differences between the two streams of the working fluid exchanging heat in the high-temperature heat exchanger. This effeet is of basically significant importance for the instant method as within the range of pressures approaching the critical pressure there is observed a considerable variation of the specific heat with changes of temperature. In order to cope with the above problem, in accordance with the present invention, the high temperature heat exchanger is divided into a number of sections characterized by a varying ratio of mass flow rates of the two streams of the working fluid, i.e., the heating and the heated ones.

It is particularly advantageous to carry out the heat exchanging process in the high temperature heat exchanger with a constant mass flow rate of the heated stream while varying in a stepped fashion the mass flow rate of the heating medium when passing from one section of the high temperature heat exchanger to the next one. In accordance with another advantageous feature of the present invention, a part of the heating stream when passing from one section of the high temperature heat exchanger to another is led from the heat exchanger and cooled by extracting a part of its heat for heating purposes within the power cycle itself or outside the cycle, and thereafter this cooled part of the medium is directed again into the next section of the heat exchanger.

As a result of the varying mass flow rate of the heating medium within the high temperature heat exchanger, in accordance with the present invention the method enables obtaining an approximately constant temperature difference between the two heat exchanging streams of the working medium and consequently optimal conditions for the heat exchanging process and the attainable efficiency of the power cycle.

The method in accordance with the present invention is described hereafter by way of example, in a more detailed manner with reference to the appended drawing, wherein:

FIG. 1 is a schematic arrangement of a part of a steam turbine power cycle provided with a high temperature heat exchanger divided into five sections, and

FIG. 2 is a schematic arrangement of the same part of the power cycle as in FIG. 1 but with some part of the heating stream being carried away from the heat exchanger.

As shown in FIG. 1, steam at supercritical initial conditions is expanded in turbine T from state 1 to state 2 the pressure still being higher than the critical pressure. From here the stream is divided into two basic streams one of which, beginning from point 11 is directed to an interstage resuperheater and subsequently to the next turbine (which is not shown in the drawing). The other stream is directed to the high temperature regenerative heat exchanger RWC serving thereat as the heating medium. The high temperature regenerative heat exchanger RWC is divided into five sections denoted as A, B, C, D and E. When passing from one of the above sections to the next one the rate of flow of the heating medium is changed in stepped fashion. The above mentioned changes are effected either by bleeding at certain amount of the heating medium as for instance between sections B and C where a part of the medium leaving section B is directed to point 11 and further to the interstage resuperheater, or the said changes are effected by directing a suitable amount of the heating medium from another point of the power cycle to a given section of the high temperature regenerative heat exchanger as occurs between sections A and B, C and D, as well as D and E. In the case when additional amounts of the heating medium are directed from outside of the preceeding section of the regenerative heat exchanger, at the inlet of a given section there are mixed two streams of the working fluid in different thermal states and as a result a cycle is obtained in which the temperatures of the heating medium at the outlet of one section differs from the temperatures of the heating medium at the inlet of the next section. In the example described these conditions exist between sections A and B, C and D as well as D and E. Taking also into consideration the effect of bleeding a certain amount of the working fluid leaving section B, a situation arises in which between any two consecutive sections of the regenerative heat exchanger RWC the mass flow rate of the beating medium is being changed in a stepped fashion. The greater the number of sections into which the regenerative heat exchanger RWC is divided, the smaller are the variations of the temperature differences between two streams of the working medium exhanging heat in the heat exchanger RWC. Consequently, the state of the steam directed from turbine T to the interstage resuperheater differs from the state of the steam immediately behind the said turbine T, at point 2 which determines the conditions of the heating medium at the inlet to the regenerative heat exchanger RWC.

The stream of the heating medium leaving the regenerative heat exchanger RWC at point 9 is passed through pump P in which the pressure of the working fluid is increased to state 10 enabling the said stream of working fluid to mix with the stream of feed water leaving the feed water heater W at point 6. As a result ofthe above mixing process the stream of the working fluid attains condition 7, and after passing through the regenerative heat exchanger RWC the heated medium attains conditions 8, whereafter the stream of working fluid is directed into the boiler K.

In the embodiment in FIG. 2, the high temperature regenerative heat exchanger RWC is divided into three sections F, G and H. The first section F and the last one H are characterized by the same value of mass flow rate of the heating medium. In the central section G the mass flow rate is reduced by the amount of the working fluid bled at the outlet of section F with subsequent extraction of the amount of heat Q for heating purposes either within the power cycle or outside the cycle. The cooled part of the stream of working fluid is afterwards directed again into the regenerative heat exchanger RWC at the inlet of section H. Consequently stepped changes of the mass flow rate of the heating medium are obtained within the regenerative heat exchanger RWC, and simultaneously the possibility of producing mechanical energy in parallel with heat for technological purposes leads to a considerable improvement of the efficiency of the power cycle.

What is claimed is:

l. A method of improving the efficiency of the power cycle of a steam turbine for initial supercritical steam conditions comprising the steps of expanding the working fluid from the initial supercritical steam conditions to a lower pressure which is higher than the critical pressure; dividing the working fluid into two streams; cooling one stream in a high temperature regenerative heat exchanger as a heating stream for yielding its heat to a feed water stream passing through the heat exchanger and constituting a heated stream; further expanding the other of said two streams in the turbine; mixing thereafter said one stream after it has passed through the exchanger with said other stream after it has been further expanded to constitute the feed water stream entering the regenerative heat exchanger;

the high temperature regenerative heat exchanger being divided into a number of successive sections and varying the ratio of the mass flow rates of the heated stream to the heating stream in successive sections in said heat exchanger in stepped manner to provide substantially constant temperature differences between the two streams in the sections of the heat exchanger by selectively dividing a part of one of the streams from one section of the heat exchanger to another.

2. A method as claimed in claim 1 wherein the ratio of the mass flow rates is varied by stepwise varying the mass flow rate of the heating stream through the successive sections of the heat exchanger.

3. A method as claimed in claim 1 wherein the mass flow rate of the heating stream is stepwise varied by directing a part of the heating stream externally of the heat exchanger from one section to another and bypassing an intermediate section therebetween.

4. A method as claimed in claim 3 comprising cooling the externally directed part of the heating stream before it is returned to the heat exchanger. 

1. A method of improving the efficiency of the power cycle of a steam turbine for initial supercritical steam conditions comprising the steps of expanding the working fluid from the initial supercritical steam conditions to a lower pressure which is higher than the critical pressure; dividing the working fluid into two streams; cooling one stream in a high temperature regenerative heat exchanger as a heating stream for yielding its heat to a feed water stream passing through the heat exchanger and constituting a heated stream; further expanding the other of said two streams in the turbine; mixing thereafter said one stream after it has passed through the exchanger with said other stream after it has been further expanded to constitute the feed water stream entering the regenerative heat exchanger; the high temperature regenerative heat exchanger being divided into a number of successive sections and varying the ratio of the mass flow rates of the heated stream to the heating stream in successive sections in said heat exchanger in stepped manner to provide substantially constant temperature differences between the two streams in the sections of the heat exchanger by selectively dividing a part of one of the streams from one section of the heat exchanger to another.
 2. A method as claimed in claim 1 wherein the ratio of the mass flow rates is varied by stepwise varying the mass flow rate of the heating stream through the successive sections of the heat exchanger.
 3. A method as claimed in claim 1 wherein the mass flow rate of the heating stream is stepwise varied by directing a part of the heating stream externally of the heat exchanger from one section to another and by-passing an intermediate section therebetween.
 4. A method as claimed in claim 3 comprising cooling the externally directed part of the heating stream before it is returned to thE heat exchanger. 